Duct burner including a perforated flame holder

ABSTRACT

A duct burner includes a perforated flame holder and a fuel header that is spaced away from the perforated flame holder and has a plurality of fuel nozzles. Each of the fuel nozzles is arranged to emit a fuel stream toward a respective portion of the perforated flame holder. A combustion reaction supported by the fuel streams is held within a plurality of apertures extending through the perforated flame holder.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority benefit from U.S. Provisional Patent Application No. 62/117,399, entitled “DUCT BURNER INCLUDING A PERFORATED FLAME HOLDER,” filed Feb. 17, 2015 (docket number 2651-250-02); which, to the extent not inconsistent with the disclosure herein, is incorporated by reference.

SUMMARY

According to an embodiment, a duct burner includes a fuel rail configured to cooperate with air flowing through an airflow space defined by a duct wall to deliver a fuel and air mixture to a perforated flame holder disposed in the airflow space. The perforated flame holder holds a combustion reaction to heat an oxygen-containing gas flowing through the duct. This oxygen-containing gas may be, for example, air, air vitiated by combustion products, turbine exhaust gas, flue gas, etc. Accordingly, the term “air,” as used herein, refers to any oxygen containing gas of sufficient oxygen concentration to support combustion, unless further definition is provided. Likewise, related terms, such as airflow, airflow space, and the like, are to be interpreted accordingly, absent further definition.

According to an embodiment, a system includes a transport duct configured to carry a gaseous fluid and a burner positioned inside the transport duct. The burner includes a flame holder having a first face, a second face lying opposite the first face, and a plurality of apertures extending through the flame holder between the first and second faces; and a fuel nozzle positioned and configured to emit a fuel stream toward the first face of the perforated flame holder.

According to an embodiment, a duct burner includes a perforated flame holder having a first face, a second face lying opposite the first face, and a plurality of apertures extending through the perforated flame holder between the first and second faces, the first face having an aspect ratio of greater than 2:1. A fuel header is spaced away from, and extending substantially parallel to, a long dimension of the first face of the perforated flame holder. A plurality of fuel nozzles are distributed along the fuel header, each configured to emit a fuel stream toward a respective portion of the first face of the perforated flame holder.

According to an embodiment, a method includes supporting a combustion reaction substantially within a plurality of apertures extending through a perforated flame holder positioned within a fluid transport duct by introducing a fuel stream to a first face of the perforated flame holder and transferring heat produced by the combustion reaction to a gaseous fluid flowing in the transport duct past the perforated flame holder.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a duct burner that includes a perforated flame holder, according to an embodiment.

FIG. 2 is a simplified perspective view of a duct burner system including a perforated flame holder, according to an embodiment.

FIG. 3 is a side sectional diagram of a portion of the perforated flame holder of FIGS. 1 and 2, according to an embodiment.

FIG. 4 is a flow chart showing a method for operating a burner system including the perforated flame holder of FIGS. 1, 2 and 3, according to an embodiment.

FIG. 5 is a simplified perspective view of some elements of a duct burner system, according to an embodiment.

FIGS. 6A-6C are diagrammatic side-sectional views of the duct burner system of FIG. 5, according to an embodiment, taken in a plane that shows one of a plurality of main fuel nozzles and a corresponding one of a plurality of pilot nozzles, during respective stages or modes of operation.

FIGS. 7-9 are diagrammatic side-sectional views of a portion of a co-generation system that includes a duct burner system, according to respective embodiments.

DETAILED DESCRIPTION

In the following detailed description, reference is made to the accompanying drawings, which form a part hereof. In the drawings, similar symbols typically identify similar components, unless context dictates otherwise. The illustrative embodiments described in the detailed description, drawings, and claims are not meant to be limiting. Other embodiments may be utilized, and other changes may be made, without departing from the spirit or scope of the subject matter presented here.

FIG. 1 is a diagram of a duct burner system 100 that includes a perforated flame holder 102, according to an embodiment. In the duct burner 100, a duct wall 104 defines an airflow space 106 in which the burner 100 is disposed. Air, turbine exhaust gas, or other gas enters the airflow space 106 from the left and exits at the right. The duct burner 100 raises the temperature of the gas flowing through the airflow space 106.

FIG. 1 represents a sectional view of the duct burner 100, according to an embodiment. The perforated flame holder 102 extends into the paper. A fuel rail 108 also extends into the paper. The fuel rail 108 is punctuated by a plurality of orifices 110 configured to output fuel into the airflow space 106 to form a fuel and gas mixture 112 that impinges upon the perforated flame holder 102. An ignition source 114 is configured to selectively ignite the fuel after it exits the orifices 108. A controller 116 is operatively coupled to the igniter 114.

Optionally, especially for applications where the gas flowing into the airflow space contains a low oxygen concentration, an oxidant manifold (not shown) can be disposed to output oxidant, such as combustion air, adjacent to each of the plurality of fuel orifices 110 in the fuel rail 108. In some applications, the duct burner 100 can be provided to use up excess oxygen supplied output by an upstream combustion process, such as in a gas turbine combustor. In such a case, the gas entering the airflow space 106 may have sufficient oxygen to obviate the need to supplement the gas with additional oxidant, and the oxidant manifold can be omitted.

The controller 116 can be configured, upon receipt of a command via a data interface 118 to start combustion, to actuate a fuel valve 120 to initiate a fuel flow from a fuel source 122 to the fuel rail 108. In many embodiments, the preferred fuel is a hydrocarbon-based gas such as natural gas or propane. Simultaneously with or before opening the fuel valve 120, the controller 116 can enable the ignition source 114 to cause the fuel to ignite upon exiting the plurality of orifices 110. The ignition source 114 can include a hot surface igniter, a spark discharge igniter, a pilot flame igniter, or other igniter type known to those skilled in the art.

Upon ignition of the fuel, a start-up flame preheats the perforated flame holder 102 until the perforated flame holder reaches a start-up temperature. The start-up temperature may be set to be slightly higher than the autoignition temperature of the fuel to allow for a cooling effect upon receipt of cool fuel and air. Optionally, a sensor 124 can be configured to detect the temperature of the perforated flame holder 102. The controller 116 can be configured to cause the duct burner 100 to transition from a start-up mode to an operating mode upon receipt of a signal or data from the sensor 124 consistent with the perforated flame holder 102 being heated to the start-up temperature.

Upon the perforated flame holder 102 reaching the start-up temperature, the controller 116 can disable the ignition source 114. Disabling the ignition source can cause the fuel to flow without immediately combusting to form the air and fuel mixture 112. In an embodiment, the air and fuel mixture enters the perforated flame holder 102 and is ignited by the elevated start-up temperature of the perforated flame holder. Optionally, after a short period of equilibration, the perforated flame holder 102 reaches an operating temperature that may be somewhat different than the start-up temperature. The controller 116 may modulate the fuel valve 120 during this time to maintain combustion while ramping up the heat output to a desired rate.

Upon reaching a nominal selected operating state, a majority the combustion reaction can be contained within the perforated flame holder 102. The combustion reaction in the perforated flame holder 102 can be very clean, characterized by both very low production of oxides of nitrogen (commonly referred to as NOx herein) and very low output of carbon monoxide (CO).

FIG. 2 is a simplified diagram of a burner system 200 including a perforated flame holder 102 configured to hold a combustion reaction, according to an embodiment. As used herein, the terms perforated flame holder, perforated reaction holder, porous flame holder, porous reaction holder, duplex, and duplex tile shall be considered synonymous unless further definition is provided.

Experiments performed by the inventors have shown that perforated flame holders 102 described herein can support very clean combustion. Specifically, in experimental use of systems 200 ranging from pilot scale to full scale, output of oxides of nitrogen (NOx) was measured to range from low single digit parts per million (ppm) down to undetectable (less than 1 ppm) concentration of NOx at the stack. These remarkable results were measured at 3% (dry) oxygen (O₂) concentration with undetectable carbon monoxide (CO) at stack temperatures typical of industrial furnace applications (1400-1600° F.). Moreover, these results did not require any extraordinary measures such as selective catalytic reduction (SCR), selective non-catalytic reduction (SNCR), water/steam injection, external flue gas recirculation (FGR), or other heroic extremes that may be required for conventional burners to even approach such clean combustion.

According to embodiments, the burner system 200 includes a fuel and oxidant source 202 disposed to output fuel and oxidant into a combustion volume 204 to form a fuel and oxidant mixture 206. As used herein, the terms fuel and oxidant mixture and fuel stream may be used interchangeably and considered synonymous depending on the context, unless further definition is provided. As used herein, the terms combustion volume, combustion chamber, furnace volume, and the like shall be considered synonymous unless further definition is provided. The perforated flame holder 102 is disposed in the combustion volume 204 and positioned to receive the fuel and oxidant mixture 206.

FIG. 3 is a side sectional diagram 300 of a portion of the perforated flame holder 102 of FIGS. 1 and 2, according to an embodiment. Referring to FIGS. 2 and 3, the perforated flame holder 102 includes a perforated flame holder body 208 defining a plurality of perforations 210 aligned to receive the fuel and oxidant mixture 206 from the fuel and oxidant source 202. As used herein, the terms perforation, pore, aperture, elongated aperture, and the like, in the context of the perforated flame holder 102, shall be considered synonymous unless further definition is provided. The perforations 210 are configured to collectively hold a combustion reaction 302 supported by the fuel and oxidant mixture 206.

The fuel can include hydrogen, a hydrocarbon gas, a vaporized hydrocarbon liquid, an atomized hydrocarbon liquid, or a powdered or pulverized solid. The fuel can be a single species or can include a mixture of gas(es), vapor(s), atomized liquid(s), and/or pulverized solid(s). For example, in a process heater application the fuel can include fuel gas or byproducts from the process that include carbon monoxide (CO), hydrogen (H₂), and methane (CH₄). In another application the fuel can include natural gas (mostly CH₄) or propane (C₃H₈). In another application, the fuel can include #2 fuel oil or #6 fuel oil. Dual fuel applications and flexible fuel applications are similarly contemplated by the inventors. The oxidant can include oxygen carried by air, flue gas, and/or can include another oxidant, either pure or carried by a carrier gas. The terms oxidant and oxidizer shall be considered synonymous herein.

According to an embodiment, the perforated flame holder body 208 can be bounded by an input face 212 disposed to receive the fuel and oxidant mixture 206, an output face 214 facing away from the fuel and oxidant source 202, and a peripheral surface 216 defining a lateral extent of the perforated flame holder 102. The plurality of perforations 210 which are defined by the perforated flame holder body 208 extend from the input face 212 to the output face 214. The plurality of perforations 210 can receive the fuel and oxidant mixture 206 at the input face 212. The fuel and oxidant mixture 206 can then combust in or near the plurality of perforations 210 and combustion products can exit the plurality of perforations 210 at or near the output face 214.

According to an embodiment, the perforated flame holder 102 is configured to hold a majority of the combustion reaction 302 within the perforations 210. For example, on a steady-state basis, more than half the molecules of fuel output into the combustion volume 204 by the fuel and oxidant source 202 may be converted to combustion products between the input face 212 and the output face 214 of the perforated flame holder 102. According to an alternative interpretation, more than half of the heat or thermal energy output by the combustion reaction 302 may be output between the input face 212 and the output face 214 of the perforated flame holder 102. As used herein, the terms heat, heat energy, and thermal energy shall be considered synonymous unless further definition is provided. As used above, heat energy and thermal energy refer generally to the released chemical energy initially held by reactants during the combustion reaction 302. As used elsewhere herein, heat, heat energy and thermal energy correspond to a detectable temperature rise undergone by real bodies characterized by heat capacities. Under nominal operating conditions, the perforations 210 can be configured to collectively hold at least 80% of the combustion reaction 302 between the input face 212 and the output face 214 of the perforated flame holder 102. In some experiments, the inventors produced a combustion reaction 302 that was apparently wholly contained in the perforations 210 between the input face 212 and the output face 214 of the perforated flame holder 102. According to an alternative interpretation, the perforated flame holder 102 can support combustion between the input face 212 and output face 214 when combustion is “time-averaged.” For example, during transients, such as before the perforated flame holder 102 is fully heated, or if too high a (cooling) load is placed on the system, the combustion may travel somewhat downstream from the output face 214 of the perforated flame holder 102. Alternatively, if the cooling load is relatively low and/or the furnace temperature reaches a high level, the combustion may travel somewhat upstream of the input face 212 of the perforated flame holder 102.

While a “flame” is described in a manner intended for ease of description, it should be understood that in some instances, no visible flame is present. Combustion occurs primarily within the perforations 210, but the “glow” of combustion heat is dominated by a visible glow of the perforated flame holder 102 itself. In other instances, the inventors have noted transient “huffing” or “flashback” wherein a visible flame momentarily ignites in a region lying between the input face 212 of the perforated flame holder 102 and the fuel nozzle 218, within the dilution region D_(D). Such transient huffing or flashback is generally short in duration such that, on a time-averaged basis, a majority of combustion occurs within the perforations 210 of the perforated flame holder 102, between the input face 212 and the output face 214. In still other instances, the inventors have noted apparent combustion occurring downstream from the output face 214 of the perforated flame holder 102, but still a majority of combustion occurred within the perforated flame holder 102 as evidenced by continued visible glow from the perforated flame holder 102 that was observed.

The perforated flame holder 102 can be configured to receive heat from the combustion reaction 302 and output a portion of the received heat as thermal radiation 304 to heat-receiving structures (e.g., furnace walls and/or radiant section working fluid tubes) in or adjacent to the combustion volume 204. As used herein, terms such as radiation, thermal radiation, radiant heat, heat radiation, etc. are to be construed as being substantially synonymous, unless further definition is provided. Specifically, such terms refer to blackbody-type radiation of electromagnetic energy, primarily at infrared wavelengths, but also at visible wavelengths owing to elevated temperature of the perforated flame holder body 208.

Referring especially to FIG. 3, the perforated flame holder 102 outputs another portion of the received heat to the fuel and oxidant mixture 206 received at the input face 212 of the perforated flame holder 102. The perforated flame holder body 208 may receive heat from the combustion reaction 302 at least in heat receiving regions 306 of perforation walls 308. Experimental evidence has suggested to the inventors that the position of the heat receiving regions 306, or at least the position corresponding to a maximum rate of receipt of heat, can vary along the length of the perforation walls 308. In some experiments, the location of maximum receipt of heat was apparently between ⅓ and ½ of the distance from the input face 212 to the output face 214 (i.e., somewhat nearer to the input face 212 than to the output face 214). The inventors contemplate that the heat receiving regions 306 may lie nearer to the output face 214 of the perforated flame holder 102 under other conditions. Most probably, there is no clearly defined edge of the heat receiving regions 306 (or for that matter, the heat output regions 310, described below). For ease of understanding, the heat receiving regions 306 and the heat output regions 310 will be described as particular regions 306, 310.

The perforated flame holder body 208 can be characterized by a heat capacity. The perforated flame holder body 208 may hold thermal energy from the combustion reaction 302 in an amount corresponding to the heat capacity multiplied by temperature rise, and transfer the thermal energy from the heat receiving regions 306 to heat output regions 310 of the perforation walls 308. Generally, the heat output regions 310 are nearer to the input face 212 than are the heat receiving regions 306. According to one interpretation, the perforated flame holder body 208 can transfer heat from the heat receiving regions 306 to the heat output regions 310 via thermal radiation, depicted graphically as 304. According to another interpretation, the perforated flame holder body 208 can transfer heat from the heat receiving regions 306 to the heat output regions 310 via heat conduction along heat conduction paths 312. The inventors contemplate that multiple heat transfer mechanisms including conduction, radiation, and possibly convection may be operative in transferring heat from the heat receiving regions 306 to the heat output regions 310. In this way, the perforated flame holder 102 may act as a heat source to maintain the combustion reaction 302, even under conditions where a combustion reaction 302 would not be stable when supported from a conventional flame holder.

The inventors believe that the perforated flame holder 102 causes the combustion reaction 302 to begin within thermal boundary layers 314 formed adjacent to walls 308 of the perforations 210. Insofar as combustion is generally understood to include a large number of individual reactions, and since a large portion of combustion energy is released within the perforated flame holder 102, it is apparent that at least a majority of the individual reactions occur within the perforated flame holder 102. As the relatively cool fuel and oxidant mixture 206 approaches the input face 212, the flow is split into portions that respectively travel through individual perforations 210. The hot perforated flame holder body 208 transfers heat to the fluid, notably within thermal boundary layers 314 that progressively thicken as more and more heat is transferred to the incoming fuel and oxidant mixture 206. After reaching a combustion temperature (e.g., the auto-ignition temperature of the fuel), the reactants continue to flow while a chemical ignition delay time elapses, over which time the combustion reaction 302 occurs. Accordingly, the combustion reaction 302 is shown as occurring within the thermal boundary layers 314. As flow progresses, the thermal boundary layers 314 merge at a merger point 316. Ideally, the merger point 316 lies between the input face 212 and output face 214 that define the ends of the perforations 210. At some position along the length of a perforation 210, the combustion reaction 302 outputs more heat to the perforated flame holder body 208 than it receives from the perforated flame holder body 208. The heat is received at the heat receiving region 306, is held by the perforated flame holder body 208, and is transported to the heat output region 310 nearer to the input face 212, where the heat is transferred into the cool reactants (and any included diluent) to bring the reactants to the ignition temperature.

In an embodiment, each of the perforations 210 is characterized by a length L defined as a reaction fluid propagation path length between the input face 212 and the output face 214 of the perforated flame holder 102. As used herein, the term reaction fluid refers to matter that travels through a perforation 210. Near the input face 212, the reaction fluid includes the fuel and oxidant mixture 206 (optionally including nitrogen, flue gas, and/or other “non-reactive” species). Within the combustion reaction region, the reaction fluid may include plasma associated with the combustion reaction 302, molecules of reactants and their constituent parts, any non-reactive species, reaction intermediates (including transition states), and reaction products. Near the output face 214, the reaction fluid may include reaction products and byproducts, non-reactive gas, and excess oxidant.

The plurality of perforations 210 can be each characterized by a transverse dimension D between opposing perforation walls 308. The inventors have found that stable combustion can be maintained in the perforated flame holder 102 if the length L of each perforation 210 is at least four times the transverse dimension D of the perforation. In other embodiments, the length L can be greater than six times the transverse dimension D. For example, experiments have been run where L is at least eight, at least twelve, at least sixteen, and at least twenty-four times the transverse dimension D. Preferably, the length L is sufficiently long for thermal boundary layers 314 to form adjacent to the perforation walls 308 in a reaction fluid flowing through the perforations 210 to converge at merger points 316 within the perforations 210 between the input face 212 and the output face 214 of the perforated flame holder 102. In experiments, the inventors have found L/D ratios between 12 and 48 to work well (i.e., produce low NOx, produce low CO, and maintain stable combustion).

The perforated flame holder body 208 can be configured to convey heat between adjacent perforations 210. The heat conveyed between adjacent perforations 210 can be selected to cause heat output from the combustion reaction portion 302 in a first perforation 210 to supply heat to stabilize a combustion reaction portion 302 in an adjacent perforation 210.

Referring especially to FIG. 2, the fuel and oxidant source 202 can further include a fuel nozzle 218, configured to output fuel, and an oxidant source 220 configured to output a fluid including the oxidant. For example, the fuel nozzle 218 can be configured to output pure fuel. The oxidant source 220 can be configured to output combustion air carrying oxygen, and optionally, flue gas.

The perforated flame holder 102 can be held by a perforated flame holder support structure 222 configured to hold the perforated flame holder 102 at a dilution distance D_(D) away from the fuel nozzle 218. The fuel nozzle 218 can be configured to emit a fuel jet selected to entrain the oxidant to form the fuel and oxidant mixture 206 as the fuel jet and oxidant travel along a path to the perforated flame holder 102 through the dilution distance D_(D) between the fuel nozzle 218 and the perforated flame holder 102. Additionally or alternatively (particularly when a blower is used to deliver oxidant contained in combustion air), the oxidant or combustion air source can be configured to entrain the fuel and the fuel and oxidant travel through the dilution distance D_(D). In some embodiments, a flue gas recirculation path 224 can be provided. Additionally or alternatively, the fuel nozzle 218 can be configured to emit a fuel jet selected to entrain the oxidant and to entrain flue gas as the fuel jet travels through the dilution distance D_(D) between the fuel nozzle 218 and the input face 212 of the perforated flame holder 102.

The fuel nozzle 218 can be configured to emit the fuel through one or more fuel orifices 226 having an inside diameter dimension that is referred to as “nozzle diameter.” The perforated flame holder support structure 222 can support the perforated flame holder 102 to receive the fuel and oxidant mixture 206 at the distance D_(D) away from the fuel nozzle 218 greater than 20 times the nozzle diameter. In another embodiment, the perforated flame holder 102 is disposed to receive the fuel and oxidant mixture 206 at the distance D_(D) away from the fuel nozzle 218 between 100 times and 1100 times the nozzle diameter. Preferably, the perforated flame holder support structure 222 is configured to hold the perforated flame holder 102 at a distance about 200 times or more of the nozzle diameter away from the fuel nozzle 218. When the fuel and oxidant mixture 206 travels about 200 times the nozzle diameter or more, the mixture is sufficiently homogenized to cause the combustion reaction 302 to produce minimal NOx.

The fuel and oxidant source 202 can alternatively include a premix fuel and oxidant source, according to an embodiment. A premix fuel and oxidant source can include a premix chamber (not shown), a fuel nozzle configured to output fuel into the premix chamber, and an oxidant (e.g., combustion air) channel configured to output the oxidant into the premix chamber. A flame arrestor can be disposed between the premix fuel and oxidant source and the perforated flame holder 102 and be configured to prevent flame flashback into the premix fuel and oxidant source.

The oxidant source 220, whether configured for entrainment in the combustion volume 204 or for premixing, can include a blower configured to force the oxidant through the fuel and oxidant source 202.

The support structure 222 can be configured to support the perforated flame holder 102 from a floor or wall (not shown) of the combustion volume 204, for example. In another embodiment, the support structure 222 supports the perforated flame holder 102 from the fuel and oxidant source 202. Alternatively, the support structure 222 can suspend the perforated flame holder 102 from an overhead structure (such as a flue, in the case of an up-fired system). The support structure 222 can support the perforated flame holder 102 in various orientations and directions.

The perforated flame holder 102 can include a single perforated flame holder body 208. In another embodiment, the perforated flame holder 102 can include a plurality of adjacent perforated flame holder sections that collectively provide a tiled perforated flame holder 102.

The perforated flame holder support structure 222 can be configured to support the plurality of perforated flame holder sections. The perforated flame holder support structure 222 can include a metal superalloy, a cementatious, and/or ceramic refractory material. In an embodiment, the plurality of adjacent perforated flame holder sections can be joined with a fiber reinforced refractory cement.

The perforated flame holder 102 can have a width dimension W between opposite sides of the peripheral surface 216 at least twice a thickness dimension T between the input face 212 and the output face 214. In another embodiment, the perforated flame holder 102 can have a width dimension W between opposite sides of the peripheral surface 216 at least three times, at least six times, or at least nine times the thickness dimension T between the input face 212 and the output face 214 of the perforated flame holder 102.

In an embodiment, the perforated flame holder 102 can have a width dimension W less than a width of the combustion volume 204. This can allow the flue gas circulation path 224 from above to below the perforated flame holder 102 to lie between the peripheral surface 216 of the perforated flame holder 102 and the combustion volume wall (not shown).

Referring again to both FIGS. 2 and 3, the perforations 210 can be of various shapes. In an embodiment, the perforations 210 can include elongated squares, each having a transverse dimension D between opposing sides of the squares. In another embodiment, the perforations 210 can include elongated hexagons, each having a transverse dimension D between opposing sides of the hexagons. In yet another embodiment, the perforations 210 can include hollow cylinders, each having a transverse dimension D corresponding to a diameter of the cylinder. In another embodiment, the perforations 210 can include truncated cones or truncated pyramids (e.g., frustums), each having a transverse dimension D radially symmetric relative to a length axis that extends from the input face 212 to the output face 214. In some embodiments, the perforations 210 can each have a lateral dimension D equal to or greater than a quenching distance of the flame based on standard reference conditions. Alternatively, the perforations 210 may have lateral dimension D less then than a standard reference quenching distance.

In one range of embodiments, each of the plurality of perforations 210 has a lateral dimension D between 0.05 inch and 1.0 inch. Preferably, each of the plurality of perforations 210 has a lateral dimension D between 0.1 inch and 0.5 inch. For example the plurality of perforations 210 can each have a lateral dimension D of about 0.2 to 0.4 inch.

The void fraction of a perforated flame holder 102 is defined as the total volume of all perforations 210 in a section of the perforated flame holder 102 divided by a total volume of the perforated flame holder 102 including body 208 and perforations 210. The perforated flame holder 102 should have a void fraction between 0.10 and 0.90. In an embodiment, the perforated flame holder 102 can have a void fraction between 0.30 and 0.80. In another embodiment, the perforated flame holder 102 can have a void fraction of about 0.70. Using a void fraction of about 0.70 was found to be especially effective for producing very low NOx.

The perforated flame holder 102 can be formed from a fiber reinforced cast refractory material and/or a refractory material such as an aluminum silicate material. For example, the perforated flame holder 102 can be formed to include mullite or cordierite. Additionally or alternatively, the perforated flame holder body 208 can include a metal superalloy such as Inconel or Hastelloy. The perforated flame holder body 208 can define a honeycomb. Honeycomb is an industrial term of art that need not strictly refer to a hexagonal cross section and most usually includes cells of square cross section. Honeycombs of other cross sectional areas are also known.

The inventors have found that the perforated flame holder 102 can be formed from VERSAGRID® ceramic honeycomb, available from Applied Ceramics, Inc. of Doraville, S.C.

The perforations 210 can be parallel to one another and normal to the input and output faces 212, 214. In another embodiment, the perforations 210 can be parallel to one another and formed at an angle relative to the input and output faces 212, 214. In another embodiment, the perforations 210 can be non-parallel to one another. In another embodiment, the perforations 210 can be non-parallel to one another and non-intersecting. In another embodiment, the perforations 210 can be intersecting. The body 308 can be one piece or can be formed from a plurality of sections.

In another embodiment, which is not necessarily preferred, the perforated flame holder 102 may be formed from reticulated ceramic material. The term “reticulated” refers to a netlike structure. Reticulated ceramic material is often made by dissolving a slurry into a sponge of specified porosity, allowing the slurry to harden, and burning away the sponge and curing the ceramic.

In another embodiment, which is not necessarily preferred, the perforated flame holder 102 may be formed from a ceramic material that has been punched, bored or cast to create channels.

In another embodiment, the perforated flame holder 102 can include a plurality of tubes or pipes bundled together. The plurality of perforations 210 can include hollow cylinders and can optionally also include interstitial spaces between the bundled tubes. In an embodiment, the plurality of tubes can include ceramic tubes. Refractory cement can be included between the tubes and configured to adhere the tubes together. In another embodiment, the plurality of tubes can include metal (e.g., superalloy) tubes. The plurality of tubes can be held together by a metal tension member circumferential to the plurality of tubes and arranged to hold the plurality of tubes together. The metal tension member can include stainless steel, a superalloy metal wire, and/or a superalloy metal band.

The perforated flame holder body 208 can alternatively include stacked perforated sheets of material, each sheet having openings that connect with openings of subjacent and superjacent sheets. The perforated sheets can include perforated metal sheets, ceramic sheets and/or expanded sheets. In another embodiment, the perforated flame holder body 208 can include discontinuous packing bodies such that the perforations 210 are formed in the interstitial spaces between the discontinuous packing bodies. In one example, the discontinuous packing bodies include structured packing shapes. In another example, the discontinuous packing bodies include random packing shapes. For example, the discontinuous packing bodies can include ceramic Raschig ring, ceramic Berl saddles, ceramic Intalox saddles, and/or metal rings or other shapes (e.g. Super Raschig Rings) that may be held together by a metal cage.

The inventors contemplate various explanations for why burner systems including the perforated flame holder 102 provide such clean combustion.

According to an embodiment, the perforated flame holder 102 may act as a heat source to maintain a combustion reaction even under conditions where a combustion reaction would not be stable when supported by a conventional flame holder. This capability can be leveraged to support combustion using a leaner fuel-to-oxidant mixture than is typically feasible. Thus, according to an embodiment, at the point where the fuel stream 206 contacts the input face 212 of the perforated flame holder 102, an average fuel-to-oxidant ratio of the fuel stream 206 is below a (conventional) lower combustion limit of the fuel component of the fuel stream 206—lower combustion limit defines the lowest concentration of fuel at which a fuel and oxidant mixture 206 will burn when exposed to a momentary ignition source under normal atmospheric pressure and an ambient temperature of 25° C. (77° F.).

The perforated flame holder 102 and systems including the perforated flame holder 102 described herein were found to provide substantially complete combustion of CO (single digit ppm down to undetectable, depending on experimental conditions), while supporting low NOx. According to one interpretation, such a performance can be achieved due to a sufficient mixing used to lower peak flame temperatures (among other strategies). Flame temperatures tend to peak under slightly rich conditions, which can be evident in any diffusion flame that is insufficiently mixed. By sufficiently mixing, a homogenous and slightly lean mixture can be achieved prior to combustion. This combination can result in reduced flame temperatures, and thus reduced NOx formation. In one embodiment, “slightly lean” may refer to 3% O₂, i.e. an equivalence ratio of ˜0.87. Use of even leaner mixtures is possible, but may result in elevated levels of O₂. Moreover, the inventors believe perforation walls 308 may act as a heat sink for the combustion fluid. This effect may alternatively or additionally reduce combustion temperatures and lower NOx.

According to another interpretation, production of NOx can be reduced if the combustion reaction 302 occurs over a very short duration of time. Rapid combustion causes the reactants (including oxygen and entrained nitrogen) to be exposed to NOx-formation temperature for a time too short for NOx formation kinetics to cause significant production of NOx. The time required for the reactants to pass through the perforated flame holder 102 is very short compared to a conventional flame. The low NOx production associated with perforated flame holder combustion may thus be related to the short duration of time required for the reactants (and entrained nitrogen) to pass through the perforated flame holder 102.

FIG. 4 is a flow chart showing a method 400 for operating a burner system including the perforated flame holder shown and described herein. To operate a burner system including a perforated flame holder, the perforated flame holder is first heated to a temperature sufficient to maintain combustion of the fuel and oxidant mixture.

According to a simplified description, the method 400 begins with step 402, wherein the perforated flame holder is preheated to a start-up temperature, T_(S). After the perforated flame holder is raised to the start-up temperature, the method proceeds to step 404, wherein the fuel and oxidant are provided to the perforated flame holder and combustion is held by the perforated flame holder.

According to a more detailed description, step 402 begins with step 406, wherein start-up energy is provided at the perforated flame holder.

Simultaneously or following providing start-up energy, a decision step 408 determines whether the temperature T of the perforated flame holder is at or above the start-up temperature, T_(S). As long as the temperature of the perforated flame holder is below its start-up temperature, the method loops between steps 406 and 408 within the preheat step 402. In step 408, if the temperature T of at least a predetermined portion of the perforated flame holder is greater than or equal to the start-up temperature, the method 400 proceeds to overall step 404, wherein fuel and oxidant is supplied to and combustion is held by the perforated flame holder.

Step 404 may be broken down into several discrete steps, at least some of which may occur simultaneously.

Proceeding from step 408, a fuel and oxidant mixture is provided to the perforated flame holder, as shown in step 410. The fuel and oxidant may be provided by a fuel and oxidant source that includes a separate fuel nozzle and oxidant (e.g., combustion air) source, for example. In this approach, the fuel and oxidant are output in one or more directions selected to cause the fuel and oxidant mixture to be received by the input face of the perforated flame holder. The fuel may entrain the combustion air (or alternatively, the combustion air may dilute the fuel) to provide a fuel and oxidant mixture at the input face of the perforated flame holder at a fuel dilution selected for a stable combustion reaction that can be held within the perforations of the perforated flame holder.

Proceeding to step 412, the combustion reaction is held by the perforated flame holder.

In step 414, heat may be output from the perforated flame holder. The heat output from the perforated flame holder may be used to power an industrial process, heat a working fluid, generate electricity, or provide motive power, for example.

In optional step 416, the presence of combustion may be sensed. Various sensing approaches have been used and are contemplated by the inventors. Generally, combustion held by the perforated flame holder is very stable and no unusual sensing requirement is placed on the system. Combustion sensing may be performed using an infrared sensor, a video sensor, an ultraviolet sensor, a charged species sensor, thermocouple, thermopile, flame rod, and/or other combustion sensing apparatuses. In an additional or alternative variant of step 416, a pilot flame or other ignition source may be provided to cause ignition of the fuel and oxidant mixture in the event combustion is lost at the perforated flame holder.

Proceeding to decision step 418, if combustion is sensed not to be stable, the method 400 may exit to step 424, wherein an error procedure is executed. For example, the error procedure may include turning off fuel flow, re-executing the preheating step 402, outputting an alarm signal, igniting a stand-by combustion system, or other steps. If, in step 418, combustion in the perforated flame holder is determined to be stable, the method 400 proceeds to decision step 420, wherein it is determined if combustion parameters should be changed. If no combustion parameters are to be changed, the method loops (within step 404) back to step 410, and the combustion process continues. If a change in combustion parameters is indicated, the method 400 proceeds to step 422, wherein the combustion parameter change is executed. After changing the combustion parameter(s), the method loops (within step 404) back to step 410, and combustion continues.

Combustion parameters may be scheduled to be changed, for example, if a change in heat demand is encountered. For example, if less heat is required (e.g., due to decreased electricity demand, decreased motive power requirement, or lower industrial process throughput), the fuel and oxidant flow rate may be decreased in step 422. Conversely, if heat demand is increased, then fuel and oxidant flow may be increased. Additionally or alternatively, if the combustion system is in a start-up mode, then fuel and oxidant flow may be gradually increased to the perforated flame holder over one or more iterations of the loop within step 404.

Referring again to FIG. 2, the burner system 200 includes a heater 228 operatively coupled to the perforated flame holder 102. As described in conjunction with FIGS. 3 and 4, the perforated flame holder 102 operates by outputting heat to the incoming fuel and oxidant mixture 206. After combustion is established, this heat is provided by the combustion reaction 302; but before combustion is established, the heat is provided by the heater 228.

Various heating apparatuses have been used and are contemplated by the inventors. In some embodiments, the heater 228 can include a flame holder configured to support a flame disposed to heat the perforated flame holder 102. The fuel and oxidant source 202 can include a fuel nozzle 218 configured to emit a fuel stream 206 and an oxidant source 220 configured to output oxidant (e.g., combustion air) adjacent to the fuel stream 206. The fuel nozzle 218 and oxidant source 220 can be configured to output the fuel stream 206 to be progressively diluted by the oxidant (e.g., combustion air). The perforated flame holder 102 can be disposed to receive a diluted fuel and oxidant mixture 206 that supports a combustion reaction 302 that is stabilized by the perforated flame holder 102 when the perforated flame holder 102 is at an operating temperature. A start-up flame holder, in contrast, can be configured to support a start-up flame at a location corresponding to a relatively unmixed fuel and oxidant mixture that is stable without stabilization provided by the heated perforated flame holder 102.

The burner system 200 can further include a controller 230 operatively coupled to the heater 228 and to a data interface 232. For example, the controller 230 can be configured to control a start-up flame holder actuator configured to cause the start-up flame holder to hold the start-up flame when the perforated flame holder 102 needs to be preheated and to not hold the start-up flame when the perforated flame holder 102 is at an operating temperature (e.g., when T≧T_(S)).

Various approaches for actuating a start-up flame are contemplated. In one embodiment, the start-up flame holder includes a mechanically-actuated bluff body configured to be actuated to intercept the fuel and oxidant mixture 206 to cause heat-recycling and/or stabilizing vortices and thereby hold a start-up flame; or to be actuated to not intercept the fuel and oxidant mixture 206 to cause the fuel and oxidant mixture 206 to proceed to the perforated flame holder 102. In another embodiment, a fuel control valve, blower, and/or damper may be used to select a fuel and oxidant mixture flow rate that is sufficiently low for a start-up flame to be jet-stabilized; and upon reaching a perforated flame holder 102 operating temperature, the flow rate may be increased to “blow out” the start-up flame. In another embodiment, the heater 228 may include an electrical power supply operatively coupled to the controller 230 and configured to apply an electrical charge or voltage to the fuel and oxidant mixture 206. An electrically conductive start-up flame holder may be selectively coupled to a voltage ground or other voltage selected to attract the electrical charge in the fuel and oxidant mixture 206. The attraction of the electrical charge was found by the inventors to cause a start-up flame to be held by the electrically conductive start-up flame holder.

In another embodiment, the heater 228 may include an electrical resistance heater configured to output heat to the perforated flame holder 102 and/or to the fuel and oxidant mixture 206. The electrical resistance heater can be configured to heat up the perforated flame holder 102 to an operating temperature. The heater 228 can further include a power supply and a switch operable, under control of the controller 230, to selectively couple the power supply to the electrical resistance heater.

An electrical resistance heater 228 can be formed in various ways. For example, the electrical resistance heater 228 can be formed from KANTHAL® wire (available from Sandvik Materials Technology division of Sandvik AB of Hallstaham mar, Sweden) threaded through at least a portion of the perforations 210 defined by the perforated flame holder body 208. Alternatively, the heater 228 can include an inductive heater, a high-energy beam heater (e.g. microwave or laser), a frictional heater, electro-resistive ceramic coatings, or other types of heating technologies.

Other forms of start-up apparatuses are contemplated. For example, the heater 228 can include an electrical discharge igniter or hot surface igniter configured to output a pulsed ignition to the oxidant and fuel. Additionally or alternatively, a start-up apparatus can include a pilot flame apparatus disposed to ignite the fuel and oxidant mixture 206 that would otherwise enter the perforated flame holder 102. The electrical discharge igniter, hot surface igniter, and/or pilot flame apparatus can be operatively coupled to the controller 230, which can cause the electrical discharge igniter or pilot flame apparatus to maintain combustion of the fuel and oxidant mixture 206 in or upstream from the perforated flame holder 102 before the perforated flame holder 102 is heated sufficiently to maintain combustion.

The burner system 200 can further include a sensor 234 operatively coupled to the control circuit 230. The sensor 234 can include a heat sensor configured to detect infrared radiation or a temperature of the perforated flame holder 102. The control circuit 230 can be configured to control the heating apparatus 228 responsive to input from the sensor 234. Optionally, a fuel control valve 236 can be operatively coupled to the controller 230 and configured to control a flow of fuel to the fuel and oxidant source 202. Additionally or alternatively, an oxidant blower or damper 238 can be operatively coupled to the controller 230 and configured to control flow of the oxidant (or combustion air).

The sensor 234 can further include a combustion sensor operatively coupled to the control circuit 230, the combustion sensor being configured to detect a temperature, video image, and/or spectral characteristic of a combustion reaction held by the perforated flame holder 102. The fuel control valve 236 can be configured to control a flow of fuel from a fuel source to the fuel and oxidant source 202. The controller 230 can be configured to control the fuel control valve 236 responsive to input from the combustion sensor 234. The controller 230 can be configured to control the fuel control valve 236 and/or oxidant blower or damper to control a preheat flame type of heater 228 to heat the perforated flame holder 102 to an operating temperature. The controller 230 can similarly control the fuel control valve 236 and/or the oxidant blower or damper to change the fuel and oxidant mixture 206 flow responsive to a heat demand change received as data via the data interface 232.

FIG. 5 is a diagrammatic perspective view of some elements of a duct burner system 500, according to an embodiment. The duct burner system 500 includes a perforated flame holder 502 that is substantially similar in structure and operation to the perforated flame holder 102 described with reference to FIG. 2. However, the input face 212 (and output face 214) of the flame holder can have an aspect ratio that is greater than 2:1. The flame holder 502 is configured to receive at its input face 212 a fuel stream 206 from each of a plurality of fuel nozzles. The duct burner system 500 further includes a plurality of main fuel nozzles 508 and, according to an embodiment, a corresponding plurality of pilot nozzles 512, each positioned adjacent to a respective one of the main fuel nozzles 508. In the embodiment shown in FIG. 5, the duct burner system 500 includes a main fuel header 504 and a pilot runner 506. According to an embodiment, the main fuel header 504 is a manifold pipe extending substantially along the length of the perforated flame holder 502 and spaced therefrom by a selected distance. The plurality of main fuel nozzles 508 is spaced placed along the length of the main fuel header 504. Each of the plurality of main fuel nozzles 508 can be is configured to produce a fuel stream 206 having a shape and dispersion angle selected according to a position and distance D_(D) of the nozzles 508, relative to the input face of the perforated flame holder 502. In the embodiment of FIG. 5, each of the main fuel nozzles 508 includes a purpose-made nozzle tip 510. The nozzle tips 510 can be removable, permitting simplified replacement during maintenance, or can be permanently coupled to the main fuel header 504, for ease of manufacture. According to another embodiment, as shown, for example, in FIGS. 6A-6C, each of the main fuel nozzles 508 is merely an aperture formed in the main fuel header 504.

The pilot runner 506 includes a plurality of pilot nozzles 512, each positioned adjacent to a respective one of the main fuel nozzles 508. According to an embodiment, each of the plurality of pilot nozzles 512 includes a separate nozzle tip. According to another embodiment, each of the plurality of pilot nozzles 512 comprises merely an aperture formed in the pilot runner 506.

The main fuel header 504 can be positioned so that a fuel stream 206 produced by each of the main fuel nozzles 508 is distributed substantially evenly over a respective portion of the input face 212 of the perforated flame holder 502. In the embodiment shown, the main fuel header 504 is positioned such that a longitudinal axis of the main fuel header 504 lies in a plane that is normal to, and approximately centered on the input face 212 of the flame holder 502, with each of the plurality of main fuel nozzles 508 positioned to emit a fuel stream 206 centered on a respective fuel stream axis lying in the same plane. Spacing between the main fuel nozzles 508 along the main fuel header 504 can be selected so that fuel and oxidant is introduced into the input face 212 of the flame holder 502 substantially evenly along the entire length of the flame holder, with minimal overlap of the respective fuel streams 206 at the input face 212.

According to an embodiment, the pilot runner 506 extends parallel to the main fuel header 504, with the plurality of pilot nozzles 512 oriented such that a pilot stream of fuel emitted from each of the pilot nozzles 512 intersects a fuel stream 206 emitted from a respective one of the main fuel nozzles 508.

A control element 514 can be configured to control start-up and operation of the duct burner system 500, and can include a fuel input 516 at which fuel for the duct burner is supplied, a main fuel output line 518 coupled to the main fuel header 504, and a pilot fuel output line 520 coupled to the pilot runner 506. According to an embodiment, the control element 514 is configured to enable manual control, by a user, of flows of fuel to the main fuel header 504 and the pilot runner 506. According to another embodiment, the control element 514 is configured to provide automatic and independent control of start-up and/or normal operation of the duct burner system 500.

FIGS. 6A-6C are diagrammatic side-sectional views of the duct burner system 500, according to an embodiment, taken in a plane that shows one of the plurality of main fuel nozzles 508 and the corresponding one of the plurality of pilot nozzles 512, during respective stages or modes of operation. For the most part, operation of the burner system 500 will be described hereafter with reference to the portion of the system shown in FIGS. 6A-6C, i.e., a single main fuel nozzle 508, a pilot nozzle 512, and the perforated flame holder 502. It will be understood that the processes and aspects of the duct burner system 500 described with reference to the portion of the duct burner system 500 shown in FIGS. 6A-6C are substantially simultaneously present or occurring at each of the plurality of positions that include elements corresponding to those shown, along the length of the duct burner system 500.

In the embodiment shown, the main fuel nozzle 508 comprises an aperture formed the main fuel header 504. In this embodiment, a satisfactory fuel stream 206 can be produced by selection of the position, shape, and size of the aperture. As previously discussed, according to other embodiments, the main fuel nozzle 508 can include a separate nozzle tip that is removably or permanently coupled to the main fuel header 504. One of the possible advantages of embodiments that employ removable nozzle tips is that clogged, damaged or otherwise inoperable nozzles can be replaced without requiring extended shutdown periods. Additionally, with a separate nozzle tip, the shape, distribution, and/or dispersion angle of the fuel stream 206 can be more closely controlled than with an aperture permanently coupled to the main fuel header 504. On the other hand, in embodiments in which permanently coupled apertures can produce an acceptable fuel stream 206, the manufacturing costs can be significantly reduced, not only by the elimination of individual fuel tips, but also by the elimination of the machining and forming processes that are required to configure the header 504 to receive the tips.

FIG. 6A shows the duct burner system 500 in a standby mode, in which no fuel is supplied to the burner system, and no flame in present at the perforated flame holder 502. According to an embodiment, startup of the duct burner system 500 includes preheating the perforated flame holder 502 prior to introducing the fuel stream 206 to the input face 212. FIG. 6B shows the duct burner system 500 in a startup mode, during which the perforated flame holder 502 is preheated, while FIG. 6C shows the duct burner system 500 in a normal operating mode.

Referring to FIGS. 6A-6C, according to an embodiment, at the beginning of a startup procedure, fuel may be supplied to the pilot runner 506, which produces a pilot stream from the pilot nozzle 512. A pilot flame 602 may then be ignited at the pilot nozzle 512. Ignition of the pilot flame 602 can be by any appropriate means, including, for example, by an electrical spark, a heated element, etc. The pilot flames 602 of the duct burner system 500 can be ignited individually, or, where conditions permit, a single pilot flame can be ignited, with adjacent pilot flames being ignited therefrom.

Once the pilot flame 602 is ignited, fuel can be supplied to the main fuel header 504, producing the fuel stream 206 from the main fuel nozzle 508. As previously described, the pilot nozzle 512 can be positioned to produce a pilot stream that intersects the fuel stream 206. As a result, the pilot flame 602 can be oriented so that the fuel stream 206 is ignited at a position between the main fuel nozzle 508 and the perforated flame holder 502. This produces a startup flame 604 that is held at the position where it is ignited by the pilot flame 602, as shown in FIG. 6B, according to an embodiment. The startup flame 604 then heats the perforated flame holder 502.

When at least a portion of the perforated flame holder 502 is sufficiently hot, the fuel supply to the pilot runner 506 can be closed, extinguishing the pilot flame 602. Without the pilot flame 602 to hold it, the startup flame 604 can become unstable, given the velocity of the fuel stream 206, and is subsequently extinguished, or carried downstream to the perforated flame holder 502. The fuel stream 206 may then flow outward, carrying the fuel and oxidant mixture to the input face 212 of the perforated flame holder 502. When the fuel in the fuel stream 206 reaches the preheated perforated flame holder 502, the combustion reaction 302 may be ignited, and may be held substantially within the apertures of the flame holder 502, and the system may proceed in a normal operating mode, as shown in FIG. 6C.

For clarity and simplicity, the preceding description is primarily limited to the portion of the duct burner system 500 that is shown in the drawings. However, it should be understood that there are aspects of the operation of the duct burner system 500 in which individual portions do not operate independently, but are interrelated. For the most part, these aspects will we evident to those of ordinary skill in the art. However, a few will be discussed here, for illustration.

During execution of the startup procedure described above, there may be some points at which the process proceeds only after a preceding event is confirmed. For example, in the embodiment described, introduction of fuel to the main fuel header 504 is described as proceeding once the pilot flame 602 is ignited. In other words, according to an embodiment, existence of the pilot flame 602 is confirmed prior to the introduction of fuel to the main fuel header 504. Inasmuch as the main fuel header 504 may provide fuel to each of the plurality of main fuel nozzles 508, it should therefore be understood that, in the embodiment described, fuel is, preferably, not supplied to the main fuel header 504 until after the pilot flame 602 is confirmed at each of the plurality of pilot nozzles 512.

Similarly, as described, the fuel supply to the pilot runner 506 can be closed once at least a portion of the perforated flame holder 502 is sufficiently hot. It should be understood that, according to the embodiment described, the fuel supply to the pilot runner 506 is closed only after at least a portion of the perforated flame holder 502 opposite each of the main fuel nozzles 508 is sufficiently hot. In other words, it is not necessary that the entire surface of the perforated flame holder 502 be heated to a particular temperature. If a relatively small portion of the perforated flame holder 502 is adequately heated, it will ignite a combustion reaction 302 in that portion, which will thereafter heat the surrounding areas of the perforated flame holder 502 and expand until the entire perforated flame holder 502 is in full operation. However, for efficient operation, there should be an adequately preheated portion of the perforated flame holder 502 opposite each of the main fuel nozzles 508 at the time the pilot flames 602 are extinguished.

FIG. 7 is a diagrammatic side-sectional view of a portion of a cogeneration system 700 that includes a duct burner system 500, according to an embodiment. The duct burner system 500 is shown positioned within a duct 702 of the cogeneration system 700. In the example shown, the duct 702 carries exhaust gas 704 from a turbine generator of the cogeneration system 700 toward a heat recovery steam generator (HRSG), where heat from the turbine is recovered from the turbine exhaust gas (TEG) 704 and can be used, for example, to supplement the power generated by the turbine, or for other purposes that will be apparent to those skilled in the art, in light of the present disclosure. Such an arrangement can improve the fuel efficiency of the cogeneration system 700. In the embodiment of FIG. 7, the duct burner system 500 is configured to further heat the TEG 704 in order to increase the output and efficiency of the HRSG.

According to an embodiment, the duct burner system 500 includes the elements described above with reference to FIGS. 5-6C, and is positioned within the duct 702 so that the direction of flow of the TEG 704 substantially corresponds to the flow of the fuel stream 206. In the present example, the duct 702 extends substantially horizontally so the duct burner system 500 is oriented with the pilot nozzles 512 positioned to emit the fuel streams 206 along respective fuel stream axes lying substantially horizontally, toward the perforated flame holder 502, while the input and output faces 212 and 214 of the perforated flame holder 502 are vertically oriented. In embodiments in which the duct 702 is oriented differently, the duct burner system 500 is oriented accordingly.

According to an embodiment, the perforated flame holder 502 is positioned in, and supported by a slide rail assembly 706, which can extend, for example, across a width of the duct 702. The TEG 704 can include combustion products produced by combustion in the turbine generator, and has a reduced level of oxygen, relative to normal ambient air. However, by proper operation of the turbine generator, the TEG 704 can be controlled to retain sufficient oxygen to support the combustion reaction 302.

TEG 704 a may enter the portion of the duct 702 shown in FIG. 7 carrying heat imparted by the turbine generator. A portion of the stream of TEG 704 a is entrained by the fuel stream 206 and is carried through the perforated flame holder 502, where more of the oxygen content can be consumed in supporting the combustion reaction 302. Superheated gases emerging from the perforated flame holder 502 mix with portions of the TEG 704 a that did not pass through the perforated flame holder 502, so that a resulting TEG 704 b downstream from the perforated flame holder is significantly hotter.

In addition to the heat imparted to the gases passing through the perforated flame holder 502, heat may be emitted from the perforated flame holder 502 in the form of thermal radiation 304, which impinges on side walls of the duct 702, as well as on particles carried by the TEG 704, imparting thermal energy. A portion of this energy may also be transferred to the TEG 704 as it flows downstream.

FIG. 8 is a diagrammatic side-sectional view of a portion of a cogeneration system 800 that includes a duct burner system 802, according to an embodiment. The duct burner system 802 is similar in structure and operation to the duct burner system 500 described above, except that it includes a plurality of perforated flame holders 502, and corresponding pluralities of main fuel headers 504 and pilot runners 506.

In the embodiment shown, the perforated flame holders 502 are in a stacked array 804, in which input and output faces 212, 214 of the plurality of flame holders 502 lie in respective common planes. According to an embodiment, the stacked array 804 extends across the entire height of the duct 702, so that substantially all of the TEG 704 flowing through the duct 702 passes through one or another of the plurality of flame holders 502. The slide rail assembly 706 serves to close any spaces not occupied by the stacked array 804.

According to another embodiment, the stacked array 804 is sized, and/or the slide rail assembly 706 is configured to permit a portion of the TEG to pass around the stacked array.

FIG. 9 is a diagrammatic side sectional view of a portion of a cogeneration system 900 that includes a duct burner system 902, according to an embodiment. The burner system 902 is similar in structure and operation to the duct burner system 500 described above, except that it includes heat sinks 904 coupled to the perforated flame holder 502. In the figure, cooling fins 906 of the heat sinks 904 can be aligned with the direction of flow of the TEG 704, and so can be distinguished from the bodies of the heat sinks 904 by a dotted line. During operation of the duct burner system 902, a portion of the heat generated by the combustion reaction 302 and retained in the perforated flame holder 502 may be transferred by conduction to the heat sinks 904. Much of the TEG 704 a that does not flow through the perforated flame holder 502 may flow across the cooling fins 906 of the heat sinks 904, where it it is heated by contact with the fins.

The TEG 704 is substantially transparent to infrared radiation emitted from the perforated flame holder 502, so it is heated only indirectly, as it contacts particles and surfaces that have been heated. As a result, the heat transfer efficiency by thermal radiation to the TEG 704 can be low. Much of the thermal energy may be lost as it passes through the walls of the duct 702 to other components or to the surrounding environment. In the embodiment of FIG. 9, much of the generated heat is transferred to the heat sinks 904, where the heat transfer efficiency to the TEG 704 is much higher.

For illustration, embodiments of duct burner systems have been described above with reference to their use in TEG transfer ducts of cogeneration systems. According to other embodiments, duct burner systems are used in various other applications, including, for example, as intake preheaters of combustion systems; as air dryers, in industrial process in which combustion components in the air can be tolerated; as fume incinerators, for reducing pollutant levels inside ducts or stacks carrying combustion exhaust flows to be released to the atmosphere; etc.

While various aspects and embodiments have been disclosed herein, other aspects and embodiments are contemplated. The various aspects and embodiments disclosed herein are for purposes of illustration and are not intended to be limiting, with the true scope and spirit being indicated by the following claims. 

What is claimed is:
 1. A system, comprising: a transport duct configured to carry a gaseous fluid; and a burner positioned inside the transport duct, the burner including: a perforated flame holder having a first face, a second face lying opposite the first face, and a plurality of apertures extending through the perforated flame holder between the first and second faces, and a fuel nozzle positioned and configured to emit a fuel stream toward the first face of the perforated flame holder.
 2. The system of claim 1, wherein the fuel nozzle is one of a plurality of fuel nozzles, each positioned and configured to emit a fuel stream toward a respective portion of the first face of the perforated flame holder.
 3. The system of claim 2, comprising a plurality of pilot nozzles, each positioned adjacent to a respective one of the plurality of fuel nozzles.
 4. The system of claim 2, wherein the burner comprises a fuel header extending substantially parallel to the first face of the perforated flame holder, and wherein each of the plurality of fuel nozzles includes a respective aperture extending through a wall of the fuel header.
 5. The system of claim 4, wherein the plurality of fuel nozzles is spaced along the fuel header such that, when fuel streams are emitted by the plurality of fuel nozzles, fuel is introduced into the first face of the perforated flame holder substantially evenly along a length of the perforated flame holder.
 6. The system of claim 4, wherein each of the plurality of fuel nozzles includes a nozzle tip coupled to the fuel header over the respective aperture.
 7. The system of claim 6, wherein the nozzle tips are removable.
 8. The system of claim 6, wherein the nozzle tips are attached permanently.
 9. The system of claim 4, wherein the burner comprises a plurality of pilot nozzles, each positioned adjacent to a respective one of the plurality of fuel nozzles.
 10. The system of claim 9, wherein each of the plurality of fuel nozzles is configured to emit the corresponding fuel stream along a respective fuel stream axis, and wherein each of the plurality of pilot nozzles is configured to emit a pilot fuel stream along a pilot axis that intersects the fuel stream axis of the respective one of the plurality of fuel nozzles.
 11. The system of claim 9, comprising a pilot runner extending parallel to the fuel header, and wherein each of the plurality of pilot nozzles includes a respective aperture extending through a wall of the pilot runner.
 12. The system of claim 11, comprising a controller configured to control a first flow of fuel to the fuel header and a second flow of fuel to the pilot runner.
 13. The system of claim 9, comprising ignition means for igniting a pilot flame at one or more of the plurality of pilot nozzles.
 14. The system of claim 2, wherein the burner comprises a first burner unit that includes the perforated flame holder and the plurality of fuel nozzles, and wherein the first burner unit is one of a plurality of burner units comprised by the burner, each having a respective perforated flame holder and a respective plurality of fuel nozzles.
 15. The system of claim 14, wherein the perforated flame holder of each of the plurality of burner units includes a first face and a second face lying opposite the first face, and wherein each of the plurality of burner units is positioned such that the first face of each of the respective perforated flame holders is lying in a common plane.
 16. The system of claim 1, comprising a heat sink coupled to the perforated flame holder and including a plurality of cooling fins oriented parallel to a direction of fluid flow within the transport duct.
 17. The system of claim 1, wherein the burner is positioned such that a direction of flow of a fuel stream from the fuel nozzle toward the first face of the perforated flame holder substantially corresponds to a direction of fluid flow within the transport duct.
 18. A duct burner, comprising: a perforated flame holder having a first face, a second face lying opposite the first face, and a plurality of apertures extending through the perforated flame holder between the first and second faces, the first face having an aspect ratio of greater than 2:1; a fuel header spaced away from, and extending substantially parallel to a long dimension of the first face of the perforated flame holder; and a plurality of fuel nozzles distributed along the fuel header, each configured to emit a fuel stream toward a respective portion of the first face of the perforated flame holder.
 19. The duct burner of claim 18, wherein the plurality of fuel nozzles is spaced along the fuel header such that, when fuel streams are emitted by the plurality of fuel nozzles, fuel is introduced into the first face of the perforated flame holder substantially evenly along a length of the perforated flame holder.
 20. The duct burner of claim 18, comprising: a pilot runner extending parallel to the fuel header; and a plurality of pilot nozzles distributed along the pilot runner, each positioned adjacent to a respective one of the plurality of fuel nozzles and configured to emit a pilot stream along an axis that intersects an emission axis of the respective fuel nozzle.
 21. The duct burner of claim 20, comprising ignition means for igniting a pilot flame at one or more of the plurality of pilot nozzles.
 22. The duct burner of claim 20, comprising a controller configured to control a first flow of fuel to the fuel header and a second flow of fuel to the pilot runner.
 23. The duct burner of claim 18, wherein the perforated flame holder is one of a plurality of perforated flame holders comprised by the duct burner.
 24. The duct burner of claim 23, wherein each of the plurality of perforated flame holders is arranged with a respective first face lying in a common plane.
 25. The duct burner of claim 23, wherein the fuel header is one of a plurality of fuel headers comprised by the duct burner, each spaced away from, and extending substantially parallel to a long dimension of a first face of a respective one of the plurality of perforated flame holders.
 26. A method, comprising: supporting a combustion reaction substantially within a plurality of apertures extending through a perforated flame holder positioned within a fluid transport duct by introducing a fuel stream to a first face of the perforated flame holder; and transferring heat produced by the combustion reaction to a gaseous fluid flowing in the transport duct past the perforated flame holder.
 27. The method of claim 26, wherein the supporting a combustion reaction by introducing fuel stream to a first face of the perforated flame holder comprises supporting the combustion reaction by introducing a plurality of fuel streams to respective portions of the first face of the perforated flame holder.
 28. The method of claim 27, wherein the supporting a combustion reaction by introducing a plurality of fuel streams to respective portions of the first face of the perforated flame holder comprises emitting each of the plurality of fuel streams from a respective one of a plurality of fuel nozzles distributed along a length of a fuel header that is spaced away from, and lying parallel to the first face of the perforated flame holder.
 29. The method of claim 28, comprising, prior to supporting the combustion reaction, preheating the perforated flame holder.
 30. The method of claim 29, wherein the preheating the perforated flame holder comprises holding a respective preheat flame in each of the plurality of fuel streams between the respective fuel nozzle and the first face of the perforated flame holder.
 31. The method of claim 30, wherein the holding a respective preheat flame in each of the plurality of fuel streams comprises supporting a respective pilot flame adjacent to each of the plurality of fuel streams.
 32. The method of claim 31, wherein the supporting a respective pilot flame adjacent to each of the plurality of fuel streams comprises emitting each of the pilot flames from a respective one of a plurality of pilot nozzles distributed along a length of a pilot runner extending parallel to the fuel header.
 33. The method of claim 26, wherein the supporting a combustion reaction substantially within a first plurality of apertures extending through a perforated flame holder comprises supporting a first combustion reaction substantially within the plurality of apertures extending through the perforated flame holder, the method further comprising supporting a second combustion reaction substantially within a second plurality of apertures extending through a second perforated flame holder.
 34. The method of claim 33, wherein the transferring heat produced by the combustion reaction to a gaseous fluid comprises transferring heat produced by the first combustion reaction to a gaseous fluid flowing in the transport duct past the first perforated flame holder, the method further comprising transferring heat produced by the second combustion reaction to a gaseous fluid flowing in the transport duct past the second perforated flame holder. 